Recently, small electronic endoscopes which use a solid-state image pick-up element, such as a CCD (charge-coupled device), have become popular. Such small electronic endoscopes enable many people to observe objects such as lesions within a body-cavity by viewing on a display such as a T.V. monitor detected images that are obtained using the electronic endoscope. This has the great advantage of allowing observation and diagnosis by a team of doctors, as well as enabling the patient to also observe what is being viewed using the endoscope.
Among such endoscopes, a so-called “magnifying-type endoscope” has attracted recent attention. Such an endoscope enables better diagnosing of the degree of infiltration (termed the ‘resection range’) of minute lesions as well as the observation of their micro structures.
Prior art examples of optical systems for magnifying-type endoscopes are disclosed in Japan Tokkyo S61-44283, and in Japan Kokai H4-218102. These optical systems have three or four lens groups, respectively. The magnification can be changed in order to observe lesion sites at an even higher magnification by axially displacing a specific lens element within the optical systems.
A method for driving lenses in a conventional magnification-type endoscope is shown in FIG. 12. As illustrated in FIG. 12, a frame 1 for slidably holding a negative lens element or a negative cemented lens 3 in the optical system is combined with a wire 5, and the negative lens element or negative cemented lens may be moved in directions along the optical axis by pushing or pulling on the wire. This enables one to vary not only the magnification of the optical system, but also the working distance of the optical system and thus allows for variability in the range of observation.
On the other hand, more recently, a study using a so called MEMS (Micro Electro-mechanical Systems) technique for realizing micro actuator elements has been advanced by using a semiconductor manufacturing process. An example of utilizing this technique is shown in FIG. 13, which shows a cross-sectional view of a variable-focus concave mirror. In FIG. 13, a glass substrate is etched within a circular area so as to form a cylindrical depression, and a tungsten silicide electrode is deposited on the surface of the glass substrate within the depression so as to leave an open cylindrical region above the tungsten silicide electrode. A silicon layer is affiixed to the surface of the glass substrate. The silicon layer is then etched in a region immediately above the cylindrical region so as to leave a thin layer of silicon. Thus, the thin layer of silicon spans the area above the tungsten silicide electrode. Next, a metal film is applied to the surface of the silicon layer. The metal film serves as a mirror in the circular center region and as an electrode in the circular center region and in a peripheral region. Thus, the silicon layer is supported at the peripheral region by the glass substrate, and can be deformed to different degrees by an electrostatic force between the tungsten silicide electrode and the metal electrode on the surface of the silicon. In this way, a concave mirror can be formed by the deflection of the thin silicon layer above the tungsten silicide electrode, with the focal length being adjustable depending on the voltage difference between the tungsten silicide electrode and the metal film. A variable voltage supply is provided in order to vary this difference in voltage. In this way, the working distance of an optical system which employs the concave mirror as a focus adjustment element may be varied.
A magnifying-type endoscope has a large diameter as compared to other endoscopes. In addition to the diameter needed to accommodate the lens elements in a conventional endoscope, a magnifying-type endoscope must also provide room for the wire used to adjust the magnification/working distance of the optical system. It would be very desirable, therefore, to retain the diagnostic benefits of a conventional, magnifying-type endoscope, while reducing the diameter required of such an endoscope by omitting the wire.
If, as a means for solving this problem, a variable-focus concave mirror as discussed above were to be adapted for use in an endoscope optical system as shown in FIG. 12, there would remain a drawback, in a straight path optical system, in that the radial dimension required of the concave mirror would be too large. Moreover, there would also be a problem in that the image would be a reverse image because of the single reflection. Furthermore, using the above prior art design, it would be difficult to obtain sufficient deformation of the mirror surface in order to adjust the magnification and working distance of an endoscope simply by using electrostatic forces.